Proprioceptive Mechanism for Bioinspired Fish Swimming.

In this Letter, we propose a mechanism for driving bioinspired fish swimming locomotion based on proprioceptive sensing. Proprioception provides information about and representation of a body's position, motion, and acceleration in addition to the usual five senses. We hypothesize that a feedback loop based on this "sixth" sense results in an instability, driving the locomotion. In order to test our assumptions, we use a biomimetic robot and compare the experimental results to a simple yet generic model with excellent agreement.

The fern cavitation catapult: mechanism and design principles.

Leptosporangiate ferns have evolved an ingenious cavitation catapult to disperse their spores. The mechanism relies almost entirely on the annulus, a row of 12-25 cells, which successively: (i) stores energy by evaporation of the cells' content, (ii) triggers the catapult by internal cavitation, and (iii) controls the time scales of energy release to ensure efficient spore ejection. The confluence of these three biomechanical functions within the confines of a single structure suggests a level of sophistication that goes beyond most man-made devices where specific structures or parts rarely serve more than one function. Here, we study in detail the three phases of spore ejection in the sporangia of the fern Polypodium aureum. For each of these phases, we have written the governing equations and measured the key parameters. For the opening of the sporangium, we show that the structural design of the annulus is particularly well suited to inducing bending deformations in response to osmotic volume changes. Moreover, the measured parameters for the osmoelastic design lead to a near-optimal speed of spore ejection (approx. 10 m s(-1)). Our analysis of the trigger mechanism by cavitation points to a critical cavitation pressure of approximately -100 ± 14 bar, a value that matches the most negative pressures recorded in the xylem of plants. Finally, using high-speed imaging, we elucidated the physics leading to the sharp separation of time scales (30 versus 5000 µs) in the closing dynamics. Our results highlight the importance of the precise tuning of the parameters without which the function of the leptosporangium as a catapult would be severely compromised.

The fern sporangium: a unique catapult.

Various plants and fungi have evolved ingenious devices to disperse their spores. One such mechanism is the cavitation-triggered catapult of fern sporangia. The spherical sporangia enclosing the spores are equipped with a row of 12 to 13 specialized cells, the annulus. When dehydrating, these cells induce a dramatic change of curvature in the sporangium, which is released abruptly after the cavitation of the annulus cells. The entire ejection process is reminiscent of human-made catapults with one notable exception: The sporangia lack the crossbar that arrests the catapult arm in its returning motion. We show that much of the sophistication and efficiency of the ejection mechanism lies in the two very different time scales associated with the annulus closure.

An elasto-hydrodynamical model of friction for the locomotion of Caenorhabditis elegans.

Caenorhabditis elegans (C. elegans) is one of the most studied organisms by biologists. Composed of around one thousand cells, easy to culture and to modify genetically, it is a good model system to address fundamental physiological questions and in particular to investigate neuromuscular processes. Many C. elegans mutants can be distinguished by their locomotion phenotype and it then important to understand the biomechanics of their locomotion and in particular the mechanics of their undulating crawling motion on agar aqueous gels where they are commonly grown and observed. In this article, we present a mechanical model of the friction of the worms on their substrate where we have included capillarity (which pins the worm of the gel), the hydrodynamics of the lubrication film (between worm and gel) and the substrate/body elasticity. We determine the ratio of the transverse to longitudinal friction coefficients of the worm body on the culture gel as a function of a control parameter which describes the relative role of the deformation of the gel and the viscous dissipation in the lubrication film. Experimentally this ratio is - for soft gels - larger than the maximal value predicted by our model (this maximum is equal to 2, the value for an infinite cylinder in bulk liquid) and we propose to include the plasticity of the gel (i.e. the dissipation of the deformation of the gel) for a better description of the worm/gel interaction.

Dynamics of a ball bouncing on a vibrated elastic membrane.

We investigate the dynamics of a ball bouncing on a vibrated elastic membrane. Beyond the classical solid-solid case, we study the effect of introducing new degrees of freedom by allowing substrate oscillations. The forcing frequency of the vibration strongly influences the different thresholds between the dynamical states. The simple model proposed gives good agreement between the experiments and the analytical expression for the threshold at which the ball begins to bounce. Numerical simulations permit to qualitatively recover the experimental phase diagram. Finally, we discuss how this simple system can give new insights in the recent experimental studies on bouncing droplets.

Inertial lubrication theory.

Thin fluid films can have surprising behavior depending on the boundary conditions enforced, the energy input and the specific Reynolds number of the fluid motion. Here we study the equations of motion for a thin fluid film with a free boundary and its other interface in contact with a solid wall. Although shear dissipation increases for thinner layers and the motion can generally be described in the limit as viscous, inertial modes can always be excited for a sufficiently high input of energy. We derive the minimal set of equations containing inertial effects in this strongly dissipative regime.

Taylor-like vortices in shear-banding flow of giant micelles.

Using flow visualizations in Couette geometry, we demonstrate the existence of Taylor-like vortices in the shear-banding flow of a giant micelles system. We show that vortices stacked along the vorticity direction develop concomitantly with interfacial undulations. These cellular structures are mainly localized in the induced band and their dynamics is fully correlated with that of the interface. As the control parameter increases, we observe a transition from a steady vortex flow to a state where pairs of vortices are continuously created and destroyed. Normal stress effects are discussed as potential mechanisms driving the three-dimensional flow.

Settling and swimming of flexible fluid-lubricated foils.

We study the dynamics of a flexible foil immersed in a fluid and moving close to a rigid wall. Lubrication theory allows us to derive equations of motion for the foil and thus examine the passive settling and the active swimming of a foil. This also allows us to partly answer the long-standing question in cartoon physics--can carpets fly? Our analysis suggests a region in parameter space where one may realize this dream and move the virtual towards reality.

Interface instability in shear-banding flow.

We report on the spatiotemporal dynamics of the interface in shear-banding flow of a wormlike micellar system (cetyltrimethylammonium bromide and sodium nitrate in water) during a start-up experiment. Using the scattering properties of the induced structures, we demonstrate the existence of an instability of the interface between bands along the vorticity direction. Different regimes of spatiotemporal dynamics of the interface are identified along the stress plateau. We build a model based on the flow symmetry which qualitatively describes the observed patterns.

Coarsening dynamics of the one-dimensional Cahn-Hilliard model.

The dynamics of one-dimensional Cahn-Hilliard model is studied. The stationary and particle-type solutions, the bubbles, are perused as a function of initial conditions, boundary conditions, and system size. We characterize the bubble solutions which are involved in the coarsening dynamics and establish the bifurcation scenarios of the system. A set of ordinary differential equation permits us to describe the coarsening dynamics in very good agreement with numerical simulations. We also compare these dynamics with the bubble dynamics deduced from the classical kink interaction computation where our model seems to be more appropriated. In the case of two bubbles, we deduce analytical expressions for the bubble's position and the bubble's width. Besides, a simple description of the ulterior dynamics is presented.

On the back-firing instability.

The onset of the back-firing instability is studied in a one-dimensional spatially extended and dissipative system, where propagating localized solutions become unstable. It corresponds to the emission in the tail of a solitary wave of a new wave propagating in the opposite direction. The transition is illustrated, in geometrical terms, using a model normal form equation. (c) 2004 American Institute of Physics.

Saddle-node bifurcation: appearance mechanism of pulses in the subcritical complex Ginzburg-Landau equation.

We study stationary, localized solutions in the complex subcritical Ginzburg-Landau equation in the region where there exists coexistence of homogeneous attractors. Using a matching approach, we report on the fact that the appearance of pulses are related to a saddle-node bifurcation. Numerical simulations are in good agreement with our theoretical predictions.

van der Waals-like transition in fluidized granular matter.

A phase separation of fluidized granular matter is presented. Molecular dynamics simulations of a system of grains in two spatial dimensions, with a vibrating wall and without gravity, exhibit the appearance, coalescence, and disappearance of bubbles. By identifying the mechanism responsible for the phase separation, we show that the phenomenon is analogous to the spinodal decomposition of the gas-liquid transition of the van der Waals model. We have deduced a macroscopic model for the onset of phase separation which agrees quite well with molecular dynamics simulations.

Self-parametric instability in spatially extended systems.

We study the stability of almost homoclinic homogeneous limit cycles with respect to spatiotemporal perturbations. It is shown that they are generically unstable. The instability is either the phase instability or a finite wavelength period doubling instability.

Head-on collisions of waves in an excitable FitzHugh-Nagumo system: a transition from wave annihilation to classical wave behavior.

For the particular case of an excitable FitzHugh-Nagumo system with diffusion, we investigate the transition from annihilation to crossing of the waves in the head-on collision. The analysis exploits the similarity between the local and the global phase portraits of the system. We find that the transition has features typical of the nucleation theory of first-order phase transitions, and may be understood through purely geometrical arguments. In the case of periodic boundary conditions, the transition is an infinite-dimensional analog of the creation and the vanishing of limit cycles via a homoclinic Andronov bifurcation. Both before and after the transition, the behavior of a single cell continues to be typical for excitable systems: a stable equilibrium state, and a threshold above which an excitation pulse can be induced. The generality and qualitative character of our argument shows that the phenomenon described can be observed in excitable systems well beyond the particular case presented here.

Valoración clínica y bacteriológica de netilmicina en niños con infecciones graves / Clinical and bacteriological evaluation of netilmicin in children with severe infections

Se realizó un estudio en 20 niños con infecciones graves y repercusión orgánica generalizada, para valorar la eficacia de netilmicina, nuevo agente aminoglucósido. En el estudio se incluyeron principalmente padecimentos del aparato respiratorio, del gastrointestinal y de tejidos blandos. Después de la identificación bacteriológicas y pruebas de sensibilidad a los antibióticos, se inició la administración de netilmicina a dosis de 6mg/kg/día, con rango de 9 a 100mg y una media de 27.8 mg, como dosis unitaria en tres aplicaciones diarias, por un periodo que varió de 7 a 12 días. La vía de administración fue tanto IM como IV. Durante el curso del estudio 17 pacientes mejoraron en forma importante, 8 de ellos con curación completa; 3 fallecieron, uno por pobre respuesta al tratamiento y 2 por la gravedad del padecimiento y patología concomitante de alto riesgo. La netilmicina se estudió por su menor toxicidad y por alcanzar mayor nivel sérico con menor toxicidad en comparación con gentamicina, aún cuando la seguridad de esta última es excelente